The thing Eggyhead fears most is Edgyhead, Omeletta fears her father (she was abused as a kid), BaconCat fears spray bottles.

Shadronia: spoons
Fay: sandworms

Luke fears are the demon hunters and people who pour the milk before the ceral

Boberston: Bob

Bob: socializing

And the Traveler fears the Sword of Faith

While Beldum tries to seem like he is not of afraid of anything, he is actually afraid of death

Death: You were saying?

Hoplash (bubble form): That would be failing to fulfill my duty, throwing the Nexus into a state of chaos and violence, ultimately destroying it completely.
Hoplash (regular): Hurting innocent people
Pixelsensei: The fact that I will never become a living being and replace the person I was designed around.
Coaliz: Erosion.

Knifeboi: science deniers and furries.
Thunderpug: bork. (Translation: spray bottles (they make him rusty))

sylceon: nothing
kitty losing to sylceon

Pixer fears Takis because they're too spicy for him

Does he fear being called a blue lego

No

Inkmatas: I fear losing my friends or loved ones
Peyton: I fear being ugly
Wymar: I fear nothing
Nata: I fear bioterrorists. They give me a bad reputation

Larry: losing my loved ones, and vacuums

Omeletta be like 401
401's greatest fear is Inky hating/leaving her

hasn't she done both

yeah
which is why she's an evil mastermind now

Gingerbread Man: losing my wife and son
Candystripe: nothing!
(If she was being honest, it would be not having the freedom to make her own choices)

Collector: failing to protect those I care about.

Or spiders. Spiders are the worst.

Astra: I'm not really scared of anything, I don't like bugs though.

Earth: death creeps me out.

Data: being trapped.

*Death appears behind Earth*
D: You're afraid of what?

Earth: not what I meant.

Exotic creator: She means being shut down

Earth: also not what I meant.

The Traveler: I fear the Sword of Faith